40 Gb/s 2R Burst Mode Receiver with a single
integrated SOA-MZI switch
G. T. Kanellos, N. Pleros, D. Petrantonakis, P. Zakynthinos, H. Avramopoulos
National Technical University of Athens – Photonics Communications Research Laboratory
9 Iroon Polytechniou Street, Zografou 15773 – Athens, Greece
Tel. +30-210-7722057, Fax +30-210-7722077
[email protected]
G. Maxwell and A. Poustie
Centre for Integrated Photonics, Adastral Park, Ipswich, IP5 3RE, UK
Abstract: We demonstrate a novel scheme for 2R burst mode reception
capable of operating error-free with 40 Gb/s variable length, asynchronous
optical data packets that exhibit up to 9 dB packet-to-packet power
variation. It consists of a single, hybrid integrated, SOA-based MachZehnder Interferometer (SOA-MZI) with unequal splitting ratio couplers,
configured to operate as a self-switch. We analyze theoretically the power
equalization properties of unequal splitting ratio SOA-MZI switches and
show good agreement between theory and experiment.
©2006 Optical Society of America
OCIS codes: (060.4510) Optical communications; (230.1150) All-optical devices; (250.5980)
Semiconductor Optical Amplifiers.
References and links
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1. Introduction
Multi-access networks such as Optical Burst/Packet Switching and Passive Optical Networks
(PONs) have been introduced as efficient and flexible solutions for broadband applications,
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Received 3 Jul 2006; revised 7 Sep 2006; accepted 13 Sep 2006; published 11 Apr 2007
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since they provide improved bandwidth utilization and offer a higher degree of data
transparency [1]. In these architectures data packets are transmitted asynchronously from
different network locations and travel through different optical paths, so that they exhibit large
variations in optical power on reaching the receiver end. Burst Mode receivers undertake to
handle asynchronous and unequal power level data packet streams and to ensure error free
reception and as such they are key elements for the realization of burst and packet switched
networks [2].
Specifically, the role of 2R Burst Mode Receivers is first to perform power equalization
between asynchronous data packets that arrive at the node having different power levels, and
then to provide 2R-regeneration of the input signal by means of suppressing signal
degradations induced by accumulation of noise. So far, power equalization but without 2R
regeneration has been reported utilizing optical limiting amplifier configurations [3,4]. Power
equalization and 2R regeneration of continuous 10 Gb/s data streams have been demonstrated
with a SOA-based delayed interferometer [5] and a Michelson interferometer [6]. 2R burst
mode reception has been reported with a circuit that includes a 2-element module consisting
of two gain-clamped SOAs for optical power limiting and a wavelength converter
configuration for 2R regeneration, demonstrating error-free reception of optical bursts at 40
Gb/s with packet per packet power fluctuation of 8 dB [7].
In this article we demonstrate a simple 40 Gb/s all optical 2R Burst Mode Receiver built
with a single, hybrid integrated, SOA, Mach-Zehnder Interferometer (HMZI) which has
unequal splitting ratio couplers. This unbalanced SOA-MZI exploits the non-linear transfer
function of the interferometer to suppress the ‘0’ noise level and the gain dynamics of the
SOAs to achieve packet power equalization. We analyze theoretically the power equalization
properties of this scheme by deriving a frequency domain theoretical transfer function. We
also demonstrate experimentally that it can receive asynchronous and variable length data
packets with up to 9 dB power fluctuation error free, and show good agreement with the
theoretical analysis. "Major attributes of the circuit are that it relies on standard, hybrid
fabrication technology used in commercial MZI devices and that it is simple. This is important
for a WDM burst mode receiver (BMR) where a single MZI device is required for each
wavelength channel. Fortunately, from a device perspective, multi-element MZI arrays are
becoming available [8].
2. Principle of operation and theoretical analysis
Figure 1 depicts a SOA-based MZI switch configured as a 2R Burst Mode Receiver. It
consists of an input and an output coupler with splitting ratio, a, and of two optical branches,
each one employing a SOA as the nonlinear active element. The burst-mode data packets are
inserted as the input signal into the MZI and split into two spatial components of unequal
powers aP and (1-a)P. The signal component with a power level of aP is injected into SOA1
and the second component is launched into SOA2. For successful burst-mode reception, the
two SOAs have to operate at different current conditions I1 and I2, respectively, with each
driving current determined by the corresponding SOA input signal power. For higher input
power a higher current value is required, indicating that SOA1 is driven by I1>I2, since aP>(1a)P (a denotes the higher splitting ratio). In this way, each amplifier is forced to operate in its
saturated regime for the high level packets and in its small-signal gain regime for the low level
Fig. 1. Configuration of unequal splitting ratio Mach-Zehnder Interferometer
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Received 3 Jul 2006; revised 7 Sep 2006; accepted 13 Sep 2006; published 11 Apr 2007
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packets. To this end, high gain is perceived by the low power level packets, whereas low gain
is experienced by the higher power level packets, resulting to a nearly power equalized packet
stream at the output of each SOA. In addition, the different driving conditions of the two
SOAs in combination with the unbalanced nature of MZI provide a differential phase shift
between the two spatial components, since the packet stream traveling through SOA1
experiences a higher gain value and as a result a greater phase shift than the corresponding
signal traveling through SOA2, according to the relationship φ= -(αn/2)lnG (αn is the linewidth
enhancement factor) [9]. By appropriately adjusting the two driving current values, an
approximately π differential phase shift between the two power equalized packet streams at
the two branches can be obtained, whereas the noise accompanying the signal will be
suppressed [6] as a result of the interferometric transfer function of the device. In this respect,
self-switching operation is achieved and the interference of the two spatial components at the
output coupler of the MZI yields a nearly-noise released power equalized packet stream at the
Switched port (S-port) of the MZI.
For the theoretical analysis of the self-switching operation and the packet power
equalization properties of the proposed 2R burst-mode receiver, we consider the unbalanced
MZI shown in Fig.1 with the two SOAs assumed to be two identical devices. The transfer
function of the S-port is then provided by Eq. (1)
⎛ α
G (t ) ⎞
PS (t ) = {a 2 G1 (t ) + (1 − a) 2 G2 (t ) − 2(1 − a)a G1 (t )G 2 (t ) cos⎜⎜ − n ⋅ n 2 ⎟⎟}Pin (1)
2
G1 (t ) ⎠
⎝
where Pin is the input signal optical power, αn denotes the SOA linewidth enhancement factor
and G1(t) and G2(t) are the upper and lower branch SOA gains, respectively [9].
In the case of amplitude modulated input pulse signal, the energy of the k-th individual
pulse inserted into the MZI can be expressed as
t
U ink (t ) = P0 (1 + m in ⋅ cos (Ω ⋅ k ⋅ T )) ⋅ ∫ a (t ' ) dt '
(2)
−∞
In (2), a (t ) represents the pulse waveform, P0 is the average peak power value across the
whole input signal sequence, min is the modulation depth index, Ω is the modulation
frequency and T is the pulse period. The physical representation of these parameters is
illustrated in Fig.1.
The modulation depth index mo/p at the S-output port of the MZI is obtained by
calculating the pulse peak power for every self-switched pulse. When the whole input pulse
energy has been inserted into the respective SOA, the time-dependent integral contained in (2)
can be replaced with a constant value A. The peak power of the k-th switched pulse is
obtained by replacing the integral with A, using (2) into the gain and phase relationships
provided in [10] and inserting the resulted expressions in (1). The switched pulse peak power
is then expanded into a Taylor series around the zero value of min and is expressed as a sum of
a dc power component and an oscillating, modulating power component at Ω. The amplitude
modulation depth index at the output of the gate mo/p is defined as the ratio of the modulating
power at Ω to the dc optical power. The resulting expression is a function of min , a, αn, of the
average gain values G1
min = 0
and G2
min = 0
and of the first derivatives of G1(min) and G2(min)
at min=0. For optimized self-switching performance, the average peak powers aP0 and (1-a) P0
have to correspond to an induced differential phase shift of π. This requirement leads to
G1
= G2
⋅ exp(2π α n ) . By using this condition in the fractional expression of the
min = 0
min = 0
amplitude modulation depth index mo/p, following relationship is derived:
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Received 3 Jul 2006; revised 7 Sep 2006; accepted 13 Sep 2006; published 11 Apr 2007
16 Apr 2007 / Vol. 15, No. 8 / OPTICS EXPRESS 5045
mo / p
min
= 1+
Uin
⋅
Usat
(
⎧
⎪
⎨1− G1 m=0
⎪
⎩
)⎡⎢a + a (1− a)exp⎛⎜⎜απ ⎞⎟⎟⎤⎥ + ⎡⎢1−G
3
⎢⎣
2
⎝
n
⎠⎥
⎦ ⎢
⎣
1 m=0
⎛ 2π ⎞
⎛ 2π ⎞⎤ ⎡
⎛ π ⎞⎤⎫
⎪
⎟⎥ ⋅ ⎢(1− a)3 exp⎜ ⎟ + a(1− a)2 exp⎜ ⎟⎥⎬
⎜
⎟
⎟
⎜α ⎟ (3)
α
α
⎥
⎥
⎢
n
n
n
⎝ ⎠
⎝ ⎠⎦ ⎣
⎝ ⎠⎦⎪
⎭
⋅ exp⎜⎜
⎛ 2π ⎞
⎛π ⎞
⎟ + 2a(1− a)exp⎜ ⎟
⎟
⎜α ⎟
α
⎝ n⎠
⎝ n⎠
a2 + (1− a)2 exp⎜⎜
Eq. (3) is the basic theoretical result derived within this article and provides the frequency
domain transfer function characteristics of the proposed configuration. It shows that the
relationship between mo/p and min is linear, depending on a, on αn, on the initial G1o gain value
and on the average input pulse energy Uin=PoA. The validity of Eq.(3) extends for the
parameter space for which π differential phase shift is obtained between the two
interferometer branches, which for our set of parameter values corresponds to Uin/Usat ranging
from 0 to 0.6.
Let us now define the amplitude modulation between two packets as the decimal
logarithm of the power ratio between the higher level to the lower level packet. This implies
that the amplitude modulation between the input and between the output packets depicted in
Fig. 1 can be expressed as 10 ⋅ og[(1 + min ) (1 − min )] and 10 ⋅ og (1 + mo / p ) (1 − mo / p ) ,
[
]
respectively. By replacing mo/p with its expression in (3), the calculation of the output
amplitude modulation for any given input amplitude modulation becomes possible.
Figure 2 shows the output amplitude modulation versus Uin/Usat for different coupling
ratios and input amplitude modulation of 3, 6, 10 and 13 dB. Fig. 2(a) corresponds to the case
of 50/50 splitting ratio, showing that although amplitude modulation reduction is achieved,
the output amplitude modulation remains strong. Fig. 2(b) shows results for 70/30 coupling
ratio and is almost a flat line parallel to the Uin/Usat axis, revealing that the output amplitude
modulation is reduced to less than 2 dB for a wide range of average Uin/Usat values, even when
the input amplitude modulation exceeds 10 dB. Amplitude modulation reduction of the
unbalanced MZI is even more pronounced for 90/10 splitting ratio, as shown in Fig. 2(c),
where the output amplitude modulation is always less than 1 dB. For all the theoretically
obtained graphs of Fig. 2, the αn-parameter and the initial gain value G1o were chosen to have
the typical values of 6 and 1000 respectively.
Fig. 2. Theoretically obtained graphs for mout vs Uin/Usat for coupling ratios: (a)50/50
(b) 70/30 (c) 90/10
3. Experimental Setup
Figure 3 shows the experimental setup used to evaluate the 2R burst-mode reception
capabilities of the proposed scheme. It consists of the 40 Gb/s optical packet generator, a
hybrid integrated MZI (HMZI) with unequal splitting ratio couplers that comprises the 2R
burst mode receiver, and an Electro-Absorption Modulator (EAM) used as a 40 to 10 Gb/s
demultiplexer. The input packet generator was designed to produce a stream of variable
length, asynchronous data packets exhibiting arbitrary and controllable power levels to
simulate bursty traffic. A 1553 nm DFB laser was gain switched at 10.025 Gb/s to produce 3
ps pulses after both linear and non-linear compression. A Ti:LiNbO3 electro-optic modulator
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Received 3 Jul 2006; revised 7 Sep 2006; accepted 13 Sep 2006; published 11 Apr 2007
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and a fiber bit-interleaver were used to form a 27-1 PRBS data pattern at 40.1 Gb/s. A second
Ti:LiNbO3 electro-optic modulator driven by a 3.3 Gb/s pattern generator was used to produce
a sequence of two different data packets of unequal length. This packet stream was then
launched into a third Ti:LiNbO3 modulator driven with a low rate signal so as to partially
modulate only one of the two consecutive packets and to allow the second packet through
unaffected. As a result a sequence of two data packets with unequal power levels was obtained
at the modulator output. This signal was then introduced into a split-and-delay multiplexer in
order to form asynchronous data packets. The asynchronous multiplexer consisted of a 3 dB
coupler with fiber lengths of 250 nsec differential delay at its outputs and a second 3 dB
coupler to recombine the relatively delayed signals. A variable optical delay line (ODL) and
an optical attenuator were employed in the two branches so as to be able to control the phase
alignment and the power level of the interleaved packet streams. The resulting signal was then
inserted as the input signal into the 2R burst-mode receiver, which consisted of a hybrid
integrated SOA-MZI with 70/30 input and output coupling ratios. The SOAs provided small
signal gain of 23 dB at 200 mA and 30 dB at 300 mA and their Usat parameter was 700 fJ.
Fig. 3. Experimental setup
4. Results and discussion
In order to evaluate the performance of this 2R BMR concept, streams of packets with
different power level characteristics were constructed and launched into the unequal coupling
ratio HMZI. The packet streams consisted of consecutive packets with unequal lengths of 48
and 75 bits that arrived asynchronously to the 2R BMR setup. The two SOAs of the 2R BMR
were driven with different current values of 340 mA and 190 mA. Figure 4 presents the results
with the top rows showing the input signal pulse traces and eye diagrams and the bottom rows
showing their corresponding outputs.
Figure 4(a) shows the input signal consisting of a sequence of packets whose mark value
is consecutively altered from packet to packet between 2 power levels of 9 dB difference. In
the input eye diagram, the low power level packet cannot be identified as it is masked by the
response of the 40 GHz photodiode. At the output of the 2R BMR the initial 9 dB power
difference was reduced to 2 dB and an open eye was obtained. The inner packet amplitude
modulation is due to the linear patterning effect [11] introduced by the slow SOA gain
recovery time that was greater than the bit period in both SOAs, having a value of 60 ps and
Fig. 4. Input (top row) and output (bottom row) pulse traces (timescale:1ns) and eye diagrams
(timescale:10ps) for input packets with (a) 2 power levels and (b) 4 power levels
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80ps respectively for the given SOA driving currents. In order to determine the dynamic range
of average input power levels within which this 2R BMR scheme can operate, the mark value
at the output was recorded for a broad range of average input signal pulse energies and for
packet sequences whose mark value at the input varied from packet-to-packet by 3, 6 and 9
dB. Results from these measurements are shown in Fig. 2 (b) together with the theoretically
obtained curves for output power difference versus pulse energy for a 70/30 coupling ratio
MZI switch and show good agreement between theory and experiment. The figure indicates
that for input pulse energies between 110 fJ to 390 fJ corresponding to values of Uin/Usat
between 0.15 to 0.6, the power difference is reduced from 3, 6 and 9 dB at the input, to 0.5
dB, 1dB and 2 dB respectively at the output. These results essentially imply that besides being
capable to suppress a large power variation at its input, this BMR scheme can maintain this
performance over a broad range of mean input powers. .
Fig. 5. BER curves for two demuxed channels for : 2P. level input, 4P.level input, 2P level
output, 4P. level output
The power equalization properties of the BMR were also investigated with a 4 power
level sequence of packets with differences of 3, 6 and 9 dB between them. Figure 4(b) shows
this input signal pulse trace and the corresponding eye diagram which is fully closed. Once
again at the output of the circuit the packets were power equalized within 2 dB and a clear,
open eye was obtained with current driving values of the SOAs fixed to the same values as
before. This indicates that the unequal coupling ratio HMZI can simultaneously equalize
packet sequences of multiple levels as may be expected with true bursty traffic.
For the BER measurement procedure synchronous data packets were used, generated by
properly adjusting the ODL included in the asynchronous packet multiplexer. BER
measurements were carried out for both types of input packet sequence configurations with
two and four packet power levels. An EAM was used for demultiplexing the 40 Gb/s packets
into 10 Gb/s data streams. Figure 5 shows the BER measurements for two of the four
demultiplexed 10 Gb/s channels. Input signal BER curves indicate an error floor at 10-5, even
for the maximum input power allowed on the receiver. The corresponding curves at the output
of the 2R BMR show that the error floor was eliminated and that power equalization within 2
dB provided error free operation. This was achieved without using any guardbands for
reducing the amplitude modulation imposed on the packet signal by the SOA gain excursions,
indicating that no limitations in terms of the number of consecutive ‘zeroes’ or the number of
preamble bits in the data packets are required. It should be mentioned that the BER
measurements for the 4–level signal exhibit a marginal power penalty with respect to the
corresponding 2-level signal curves, although in Fig. 4 the eye opening of 2-level output
signal appears slightly degraded compared to the 4-level output signal. This is due to the extra
OSNR degradation of the 4-level input signal induced by the additional modulator and EDFA
used for the 4-level signal generation. Finally, BER measurements were carried out also for
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Received 3 Jul 2006; revised 7 Sep 2006; accepted 13 Sep 2006; published 11 Apr 2007
16 Apr 2007 / Vol. 15, No. 8 / OPTICS EXPRESS 5048
the case of high quality power equalized data packets used as the input signal, revealing that
in this case the proposed device induces a power penalty of only 0.6 dB.
5. Conclusion
We have presented a novel scheme for 2R burst mode reception, capable at operating errorfree with 40 Gb/s variable length, asynchronous optical packets exhibiting 9 dB packet-to
packet power fluctuation. The setup is simple and requires only one SOA-MZI with unequal
splitting ratio couplers, configured as a self-switch. The theoretical analysis has confirmed the
power equalization properties of this burst mode receiver and has shown good agreement with
the experimental results.
Acknowledgments
This work was conducted within project IST-MUFINS (Contract No. 004222), supported by
the European Commission under the FP6 program.
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(C) 2007 OSA
Received 3 Jul 2006; revised 7 Sep 2006; accepted 13 Sep 2006; published 11 Apr 2007
16 Apr 2007 / Vol. 15, No. 8 / OPTICS EXPRESS 5049